One example of a conventional actuator drive circuit is shown in FIG. 3. The output terminal of an operational amplifier 52 is connected to the bases of transistors 56 and 57 through a resistor 55. The collectors of the transistors are respectively connected to a positive and a negative power supply. The emitters of the transistors are both connected to a second terminal of a load 58. The first terminal of load 58 is connected to a first terminal of a current detection resistor 59, and to the inverting input terminal of operational amplifier 52 through a feedback resistor 54. The second terminal of current detection resistor 59 is grounded. A signal input terminal 51 is connected to the inverting input terminal of operational amplifier 52 through an input resistor 53. The non-inverting input terminal of operational amplifier 52 is grounded. Since the open loop gain of operational amplifier 52 is generally very large, feedback is provided in such a manner that a signal applied to input terminal 51 is amplified by a gain determined by input resistor 53 and feedback resistor 54. The amplified signal appears at the connection point of the first terminals of load 58 and current detection resistor 59. Since the resistance Rc of current detection resistor 59 is related to the resistance R.sub.2 of feedback resistor 54 so that Rc is much less than R.sub.2, load 58 is subjected to a constant-current drive that is not dependent on the impedance characteristics of the load. However, a problem with this type of circuit is that both positive and negative power supplies are needed.
Another conventional actuator drive circuit (referred to as a bridge driver) which drives a load in a bidirectional manner using a single power supply is shown in FIG. 4. In this actuator drive circuit, a drive section 62 has a first output terminal for a positive-phase-sequence and a second output terminal for a negative-phase-sequence, and performs level shifting using a single power supply. A signal is applied to an input terminal 61, the first output terminal of the drive section is connected to the bases of transistors 63 and 66, and the second output terminal is connected to the bases of transistors 64 and 65. The emitter of transistor 63 and the collector of transistor 65 are connected to a first terminal of the load 67. The emitter of transistor 64 and the collector of transistor 66 are connected to the second terminal of load 67. The collectors of transistors 63 and 64 are connected to the single power supply, while the emitters of transistors 65 and 66 are grounded. The signal applied to input terminal 61 is subjected to level shifting by drive section 62 so that the drive circuit can operate using the single power supply. If the input signal is sinusoidal, transistors 63 and 66 are turned on and transistors 64 and 65 are turned off during the positive half cycle of the signal, and transistors 63 and 66 are turned off and transistors 64 and 65 are turned on during the negative half cycle of the signal, to drive load 67 in a bidirectional manner. Although load 67 is driven using the single power supply, this actuator drive circuit has a drawback in that the current flowing through load 67 is dependent on the impedance characteristics of the load. This is because a constant-voltage drive is provided to the load. When an inductive load, such as a linear motor actuator, is driven by the actuator drive circuit, the problem is especially serious because the inductance of the load generates a pole that adversely affects the stability of a servo system.